The present invention relates to an amplifier for amplifying oscillation signals and, more particularly, to an amplifier for amplifying a local oscillation signal of a receiving section or a carrier signal of a transmitting section and a portable telephone apparatus using this amplifier.
The communication frequency band for use in portable telephone apparatuses depends on the communication scheme used. For example, PDC (Personal Digital Cellular) operates on one of two frequency bands of 800 MHz and 1600 MHz. These frequency bands are selectively used depending on geographical regions. Therefore, if the user of the 800 MHz band telephone moves into the 1600 MHz band region for example, the telephone becomes unserviceable. Thus, it is desired to provide a portable telephone apparatus that is compatible with both the frequency bands.
Referring to FIG. 3, there is shown a portion of the constitution of the receiving section of a related-art portable telephone apparatus. In the figure, reference numeral 1 denotes an antenna, reference numeral 2 a receive signal amplifier, reference numeral 3 a mixer, reference numerals 4 and 5 local oscillators (hereinafter simply referred to as oscillators) for generating local oscillation signals, and reference numeral 6 a local oscillation signal amplifier. In the above-mentioned constitution, a receive signal received by the antenna is amplified by the amplifier 2. The amplified signal is inputted in the mixer 3. In the mixer 3, the inputted signal is mixed with a local oscillation signal inputted via the amplifier 6 to be frequency-converted into an intermediate frequency signal. The oscillators 4 and 5 are voltage-controlled oscillators that are controlled by a PLL (Phase-Locked Loop) circuit, not shown, and oscillate at different frequencies, for example 800 MHz and 1600 MHz respectively. The amplifier 6 has an amplifying transistor 7 and a parallel resonance circuit 8. The amplifying transistor 7 has a base 9 providing the input terminal, a collector 10 providing the output terminal, and an emitter 11 providing the ground terminal in a high frequency manner. Reference numerals 12 and 13 denote base bias resistors, reference numeral 14 an emitter bias resistor, reference numeral 15 a bypass capacitor for grounding the emitter 11 in a high frequency manner, and reference numeral 16 a decoupling capacitor for a power supply line 17.
One end of a microstrip line 18 and one end of a capacitor 19 are connected to the collector 10 while the other end of the microstrip line 17 and the other end of the capacitor 19 are connected to the power supply line 18 and ground respectively. The microstrip line 18 and the capacitor 19 constitute the parallel resonance circuit 8. It should be noted that the microstrip line 18 works as inductance. The power supply line 17 is applied with voltage Vcc via a power supply terminal 20. The power supply line 17 and ground are at the substantially same potential relative to high-frequency signal. Reference numerals 21 and 22 are coupling capacitors for the input and the output of the amplifier 6 respectively. Reference numeral 23 is a selector switch for selecting between the oscillators 4 and 5.
One of the oscillation frequencies of the oscillators 4 and 5 is selected by the selector switch 23. The selected output is amplified by the amplifier 6 of the next stage. The amplified signal is mixed by the mixer with a receive signal transmitted from the amplifier 2 to be frequency-converted. The converted signal is then outputted to a detector and so on in the subsequent stage.
The amplifier 6 must amplify the oscillation signals of both frequencies 800 MHz and 1600 MHz. It is difficult, however, for the amplifier 6 to amplify with a wind band from 800 MHz to 1600 MHz, so that the amplifier 6 is constituted by a tuned amplifier. Consequently, the resonance frequency of the parallel resonance circuit 8 composed of the microstrip line 18 and the capacitor 19 is set to a level generally at the center between these frequencies; 1200 MHz for example. As a result, the amplifier 6 provides the maximum amplification degree (gain) for 1200 MHz but provides a lower amplification degree for 800 MHz or 1600 MHz.
FIG. 4 shows a second related-art example. In the figure, reference numerals 24 and 25 denote amplifiers for amplifying only the oscillation signals of the oscillators 4 and 5 respectively. Reference signal 26 denotes a selector switch for switching between the output signals of the amplifiers 24 and 25. In this second example, the amplifier 24 has a parallel resonance circuit that tunes in to 800 MHz of the oscillator 4 and the amplifier 25 has a parallel resonance circuit that tunes in to 1600 MHz of the oscillator 5, these parallel resonance circuits are basically the same as the amplifier 6 of FIG. 3 in constitution. Thus, it is possible to constitute the amplifiers that have the resonance circuits suitable for these frequencies.
However, the above-mentioned related-art examples have the following problems. In the first related-art example, the amplifier 6 is used to amplify both the oscillation signals of the oscillators 4 and 5 having different frequencies, so that the resonance frequency of the parallel resonance circuit is set to 1200 MHz which is approximately at the center between the oscillation frequencies. Consequently, at the frequencies 800 MHz and 1600 MHz, the amplifier 6 is used with amplification degrees (gain) lower than the maximum level of the amplifier 6, failing to provide a sufficient amplification degree.
The bypass capacitor 15 may originally be one that has a capacity making reactance small enough relative to the resistance of the bias resistor 14 at the lower frequency 800 MHz. However, use of such a capacitor increases the reactance due to the inductance of the capacitor electrode at the higher frequency 1600 MHz, lowering the bypass effect. Hence, actually, the bypass capacitor 15 having the self resonance frequency at 1200 MHz to obtain intermediate performance.
To be more specific, as shown in the reactance characteristic diagram of FIG. 5, due to the inductance of capacitor electrode, a capacitor having a large capacity (C1) has a relatively low self resonance frequency (for example, 800 MHz) and a capacitor having a small capacity (C2) has a relatively high self resonance frequency (for example, 1600 MHz). Therefore, in the above-mentioned related-art example, a capacitor (C3) having the self resonance frequency at 1200 MHz is used as the bypass capacitor 15 as with the above-mentioned parallel resonance circuit. This, however, makes the reactance due to capacity or electrode inductance not negligible at 800 MHz or 1600 MHz to apply negative feedback to the amplifier, thereby further lowering the amplification degree. This, in turn, disables sending a local oscillation signal having enough level to the mixer 3, thereby lowering the frequency conversion gain of the mixer.
In the above-mentioned second related-art example, because the amplifiers 24 and 25 are dedicated to the oscillators 4 and 5 respectively, the problem of the first related-art example is not caused but the two amplifiers are required, which is disadvantageous in cost. Further, use of the two amplifiers prevents the portable telephone apparatus from being further reduced in size and weight.